High density element structure formed by assembly of layers and method for making same

ABSTRACT

A structure including a sequence of elements for sending or receiving a signal along an axis. Two successive elements along the direction of the axis are offset with respect to each other along the direction perpendicular to the axis. The structure includes at least two layers of material deposited on a reception substrate using the layer transfer technique. The structure particularly relates to any type of structure for which elements must have a high density, such as a print bead, networks of optical components, antenna networks, etc.

TECHNICAL FIELD AND PRIOR ART

This invention relates to a structure comprising a sequence of elementsfor sending or receiving a signal along an axis.

The invention may advantageously be used in the case in which theelements that will send or receive the signal must have a high density.For example, it may be applicable to networks of optical components(laser diodes, optical fibres, detectors), antenna networks, printheads, etc.

By way of a non-restrictive example, the invention will be describedspecifically for a print head application.

One known print technique is printing using arrays. Each array compriseselements aligned with each other side by side. Each element is either amagnetic head or a resistance, depending on whether the print signal ismagnetostatic or thermal. One or several arrays placed end to end form aline with the same width as the support to be printed.

The support on which the printing will be done moves with respect to thearrays that transform the received electrical write signals into eithermagnetic signals or thermal signals. The support is printed line by lineby relative movement of the support or the arrays. Each element receivesa write signal that is repeated for each line to be printed.

Several processes for manufacturing print heads are known according toprior art.

A first known technique consists of assembling individual print heads.For example, an individual print head may comprise a magnetic headcontrolled by a diode or a transistor. The magnetic head is made on amechanical support and the control diode is welded on the support.Individual print heads are then mounted on a mechanical support with aninsertion part between heads.

This manufacturing method limits the resolution of the print head tovalues between 150 and 300 dpi (dots per inch).

A second manufacturing method is related to microelectronics techniques.The print heads are then made collectively on a semiconductingsubstrate. One example of the collective structure of print headsobtained according to this second method is shown in FIG. 1.

The print head 1 is composed of a set of individual magnetic heads 3made on a semiconducting substrate 4. Each individual magnetic head 3 iscontrolled by a diode 2. The diodes 2 may be integrated or added ontothe semiconducting substrate 4.

Resolutions of the order of 600 dpi can be achieved using this secondmanufacturing technique. However, such resolutions cannot be reachedunless the size of the magnetic heads is reduced. Unfortunately, thisresults in a reduction in the intensity of the magnetic field induced bythe magnetic heads.

This technique also has other disadvantages.

As shown in FIG. 2, the drum 5 of the printer comes into contact withmagnetic heads 3. If the diodes 2 are added onto the substrate 4, thenthe diodes 2 need to be moved away from the heads 3 to prevent the drumfrom coming into contact with the diodes. The zone between the diodes 2and the heads 3 is then lost. The electrical resistance of the length ofthe line separating one diode 2 from a magnetic head 3 can then reducethe performances of the magnetic head.

If the diodes 2 are integrated into substrate 4, the disadvantagementioned above no longer exists. However, in this case two differentmanufacturing technologies have to be implemented; one for the diodesand another for the magnetic heads. Consequently, the manufacturingefficiency is reduced.

The invention does not have the disadvantages mentioned above.

PRESENTATION OF THE INVENTION

The invention relates to a structure comprising a sequence of elementsto send or receive a signal along an axis. At least two successiveelements along the direction of the axis are offset from each other, ina direction perpendicular to the axis.

According to the preferred embodiment of the invention, the structurecomprises at least one set of at least two layers of material depositedon a reception substrate, each layer of material comprising a sequenceof elements in line with each other along a direction parallel to theaxis, two successive elements of the structure along the directionparallel to the axis belonging to different layers.

The invention also relates to a print head comprising a sequence ofmagnetic heads or resistances to print along an axis. The print head isa structure like that according to the invention.

The invention also relates to a process for manufacturing a structurecomprising a sequence of elements to send or receive a signal along anaxis. The process comprises an assembly step of at least two materiallayers each comprising a sequence of elements aligned along a directionparallel to the axis, the assembly being made such that two successiveelements of the structure along the direction parallel to the axisbelong to two different layers.

According to the preferred embodiment of the invention, the materiallayers are assembled by transferring layers from a transfer support to areception substrate and by putting the transferred layers into bondingcontact.

The invention also relates to a process for manufacturing a print headcomprising a series of magnetic heads or resistances to make a printoutalong an axis. The process implements a process according to theinvention as described above.

The invention also relates to a process for manufacturing a structurecomprising a sequence of elements to send or receive a signal along anaxis, two successive elements along the direction of the axis beingoffset, one with respect to the other, along a direction perpendicularto the axis, characterised in that it comprises the following steps:

-   -   deposition of at least one semiconducting layer on a first        semiconducting substrate, comprising a set of elements aligned        with each other to send or receive a signal along the axis,    -   transfer from a transfer support onto a free face of a        semiconducting layer deposited on the first substrate,    -   withdrawal of the first substrate,    -   transfer using the transfer support, from the free face of the        semiconducting layer that appeared after the first substrate was        withdrawn on a first free face of a semiconducting layer fixed        on a reception substrate and comprising a set of elements        aligned with each other to send or receive a signal along the        axis,    -   molecular bonding on the two faces brought into contact by        transferring, using the transfer support.

Advantageously, the invention provides a high resolution structure. Forexample, it is possible to achieve resolutions of the order of 1200 dpior more, by successive layer transfers.

The process according to the preferred embodiment of the invention usesthe layer (or multiple layer) transfer technique by molecular bonding onwafer scale. This technique consists of making at least onesemiconducting layer on a first substrate comprising print heads, andthen transferring the semiconducting layer(s) comprising the print headsby molecular bonding, onto a second substrate on which there is alreadyat least one layer comprising print heads; thus, the number of levels ofprint heads of the structure can then be increased successively.Molecular bonding enables the manufacture of interconnections usingmicroelectronics techniques (plasma etching, sputtering). Depending onthe structures made, it is possible to bring all input/outputs onto asingle face or onto two faces. Furthermore, the layer transfer techniqueis compatible with the mixing process for meltable beads.

This technique is also compatible with wafer thinning methods, which canfurther reduce the dimensions of the print module.

The distance between two consecutive heads may be reduced to thethickness of the layers, so that a vertical pitch equivalent to thehorizontal pitch can be obtained.

This method also enables the assembly of several functions (layers forheads and layers for control circuits) and interconnections between theheads and control circuits (still using microelectronics processes).

When transferring onto a back face, the advantage of transfer bymolecular bonding means that print heads can be made on both sides suchthat the layers on each face do not come into contact with the equipmentsubstrate holders.

Compared with assembly techniques according to prior art, the assemblytechnique by transfer of layers according to the preferred embodiment ofthe invention advantageously initially makes it possible to make a stackof heads on a first support so that the precision of microelectronicstechnologies can be used, and then to transfer the stack of heads ontoanother support adapted to the required application. For example, thefirst support may be a wafer made of silicon, AsGa, SiC, SOI or InP. Forexample, the support adapted to the required application may be aplastic substrate for a print head application with a drum system (whichthen avoids deterioration of the substrate due to friction of thesubstrate on the drum), a ceramic substrate for a head using a thermaleffect, a printed circuit to connect a stack of heads to othercomponents such as control chips, or a flexible printed circuit.

The assembly technique according to the invention thus makes it possibleto integrate all functions (print heads, control circuits andinterconnections) on one or two supports depending on the variant, whileminimizing the dimensions of the module (global dimensions and distancebetween heads).

In the remainder of the description, the invention will be describedspecifically for the case in which the structure is a print head and theelements are magnetic heads. More generally, as already mentioned above,the invention relates to any structure comprising a sequence of elementsto send or receive a signal along an axis: network of optical components(laser diodes, optical fibres, detectors), antenna networks, etc.

BRIEF DESCRIPTION OF THE FIGURES

Other characteristics and advantages of the invention will become clearafter reading embodiments of the invention made with reference to theappended figures among which:

FIG. 1 shows a print head according to prior art,

FIG. 2 shows a sectional view of the print head in FIG. 1, equipped withits drum,

FIG. 3 shows a perspective view of a first embodiment of a print headaccording to the invention,

FIG. 4A shows a longitudinal sectional view of a print head and FIG. 4Bshows a transverse view of a substrate comprising print heads accordingto the invention,

FIGS. 5A to 5F show a process for manufacturing a print head accordingto the second embodiment shown in FIGS. 4A and 4B,

FIGS. 6A to 6F show different examples of print heads according to theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The same marks denote the same elements in all figures.

FIGS. 1 and 2 have already been described, and therefore there is nopoint in describing them again.

FIG. 3 shows a perspective view of two subassemblies used to make afirst example of a print head according to the invention.

A first subassembly E1 comprises a first array 6, a second array 7 andseveral blocks of diodes B1, B2 and B3. A second subassembly E2,identical to the first subassembly E1, comprises a first array 8, asecond array 9 and several blocks of diodes (not shown in the figure).

Arrays 6 and 7 of subassembly E1, and arrays 8 and 9 of subassembly E2,are fixed to each other, for example by solder beads deposited on anchorstuds. These solder beads also make an electrical connection between thearrays 6 and 7 of subassembly E1 and arrays 8 and 9 of subassembly E2.

Each array comprises a set of magnetic heads made on one of its faces.The face on which the magnetic heads of array 6 are located is fixed tothe face on which the magnetic heads of array 7 are located. Similarly,the face on which the magnetic heads of array 8 are located is fixed tothe face on which the magnetic heads of array 9 are located.

In order to fix the array 7 (or 9) to array 6 (or 8), the array 7 (or 9)is located on the array 6 (or 8), and the parts align themselves witheach other during the solder bead remelting phase.

The control diodes of the magnetic heads in the first subassembly E1 aremade in blocks (B1, B2, B3) added onto the face of the array 6 on whichthe magnetic heads are made. The diodes of blocks B1, B2, B3 are used tocontrol the magnetic heads of array 6 and the magnetic heads of array 7at the same time.

Similarly, the control diodes for the magnetic heads in the secondsubassembly E2 are made in blocks (not shown in the figure) added ontothe face of the array 8 on which the magnetic heads are made.

The diodes in the blocks thus added on are used to control the magneticheads of arrays 8 and 9.

The two subassemblies E1 and E2 are assembled to each other usingspacers T, for example rigid beads, placed in cavities C previouslyformed on the back faces of the corresponding arrays 6 and 8.

The two subassemblies E1 and E2 are pressed into contact with each otherand aligned. The remaining space between the modules can then be filledin with glue, for example epoxy resin.

The subassemblies E1 and E2 may also be assembled by meltable beadsadded onto anchor studs. These anchor studs are then firstly made on theback faces of the corresponding arrays 6 and 8.

The remelting temperature of the beads used to assemble subassemblies E1and E2 is less than the remelting temperature of the beads used forassembly of arrays 6 and 7 (or 8 and 9). The two subassemblies becomealigned with each other during the melting phase.

Regardless of the stack made, the assembly at the end of the process mayadvantageously be filled, for example, with resin to give a mechanicallyrigid assembly (similarly for other example embodiments).

The ends of the magnetic heads of subassemblies E1 and E2 form magneticpoles. By way of a non-restrictive example, the subassembly E1 comprises14 magnetic poles PE1 ₁, . . . , PE1 ₁₄ and subassembly E2 comprises 14magnetic poles PE2 ₁, . . . , PE2 ₁₄. According to the invention, thesignals that form a print line on the support are then composed, forexample, of signals output from a sequence of magnetic poles PE1 ₁, PE2₁, PE1 ₂, PE2 ₂, . . . , PE1 ₁₄, PE2 ₁₄.

The difference in height between poles in the direction perpendicular tothe axis that defines the print line may be compensated by an electronictime shift system.

By way of a non-restrictive example, for a pitch of magnetic poles thatgives a resolution of 225 dpi for an array (6, 7, 8 or 9), a resolutionof 450 dpi can be obtained for each subassembly E1, E2 by assembling thecorresponding arrays of these subassemblies with an offset of ahalf-pitch. An assembly of two sub-assemblies E1 and E2 with aquarter-pitch offset then gives a resolution 900 dpi. This resolutioncan then be further increased if the pitch of the magnetic poles in anarray gives a resolution better than 225 dpi. For example, a resolutionof 300 dpi for arrays 6, 7, 8 and 9 gives a resolution of 1200 dpi(4×300 dpi) for the print head composed of the two subassemblies E1 andE2.

An offset in the magnetic poles along a direction parallel to the printaxis may be made in several ways. For example it is possible either tomake different arrays with different positions of the magnetic poles andthen assemble the arrays symmetrically, or to make identical arrays andthen assemble the arrays offset from each other.

FIGS. 4A and 4B show a longitudinal sectional view of a print head and atransverse sectional view of a substrate comprising print headsaccording to the invention, respectively.

Substrate S1 is covered by a layer K1 that is itself covered by a layerK2. Each layer Ki (i=1, 2) is composed of a material 10 in which themagnetic heads t are formed. An increase in the resolution is obtainedby an offset of the magnetic heads t (see FIG. 4B). Advantageously, thedistance h that separates two successive magnetic heads along thedirection perpendicular to the print axis may be as high as of the orderof 20 μm to 30 μm. This value of h may be approximately equal to theminimum distance d between two successive magnetic heads in the printhead. In this case, no electronic compensation of signals applied tomagnetic heads is necessary.

The print head described in FIGS. 4A and 4B only contains two layers K1and K2 equipped with magnetic heads. However, note that more generally,the invention relates to a print head comprising at least two layersequipped with magnetic heads.

FIGS. 5A to 5F show an example of a process for manufacturing a printhead according to the invention.

FIG. 5A shows a structure composed of an initial substrate I on which astop layer 11 and a layer K2 equipped with magnetic heads t, aredeposited. The substrate I enables the formation of layers 11 and K2according to microelectronics technologies. The substrate I may forexample be a wafer of silicon, AsGa, SiC, SOI or InP. The stop layer 11may for example be a silicon oxide.

By way of a non-restrictive example, the structure shown in FIG. 5Acomprises a stop layer 11 and a layer K2 equipped with magnetic heads t.The invention also relates to the case in which there is no stop layer11 and/or the case in which the structure comprises a stack of severallayers equipped with magnetic heads.

The process for manufacturing of a print head starting from a structuresuch as that shown in FIG. 5A comprises the following steps:

-   -   transfer of a transfer support t onto the free face of the layer        K2, for example by bonding. The transfer support T, commonly        called a handle, may be made from a transparent material, for        example glass or pure silica, to avoid alignment problems (FIG.        5B),    -   withdrawal of the initial substrate I by a conventional process        or a combination of conventional processes such as mechanical        grinding, polishing, separation along a cleavage plan induced by        ionic implantation, chemical etching, reactive, selective        etching or ultrasound etching (FIG. 5C),    -   removal of the stop layer 11 (FIG. 5D),    -   transfer of the free face of layer K2 onto the free face of a        layer K1, the other face of which is fixed to a substrate S1,        and the two faces being brought into bonding contact for example        by gluing or molecular bonding; the gluing energy may be        increased by carrying out surface treatments on the surfaces to        be glued,    -   removal of the transfer support T by one of the processes        mentioned above in the description of FIG. 5C (FIG. 5F).

The process according to the invention also comprises steps in whichelectrical contact is made on the magnetic heads (opening in thepassivation and metallisation layers) These steps are not shown in thefigures.

By way of a non-restrictive example, layers K1 and K2 may be madestarting from the semiconducting material. The layer K1 can then betransferred onto layer K2 using the molecular bonding technique.Molecular bonding concerns two bonding types; hydrophilic bonding andhydrophobic bonding. Hydrophilic bonding is the result of a change to—OH interactions at the surface of the structure, for example to formSi—O—Si bonds. The forces associated with this type of interaction arestrong. The bonding energy, of the order of 100 mJ/m² at ambienttemperature, can be as high as 500 mJ/m² after annealing at 400° C. for30 minutes (values obtained by native or hydrophilic SiO2—thermalunpolished SiO2 bonding).

The bonding energy is usually determined using the strip method divulgedby W. P. MASZARA et al. in the “Bonding of silicon wafers forsilicon-on-insulator” article published in J. Appl. Phy. 64(10), Nov.15, 1988, pages 4943-4950. The bonding energy for bonding between adeposited and polished silicon oxide and a deposited and polishedsilicon oxide is of the order of 1 J/m² for annealing under the sameconditions. However, if a hydrophilic treated surface is bonded onto ahydrophobic treated surface by molecular bonding, the bonding qualityobtained is very poor and the bonding forces are very low:bonding energyof the order of 100 mJ/m² after annealing at 400° C. for 30 minutes.

With this bonding method, two substrates comprising microelectronicstechnologies can be bonded provided that their surface conditions areprepared. The precision on the pitch of two successive magnetic heads isadvantageously obtained by the bonding precision, namely ±approximately1 μm both along the direction of the print axis and along the directionperpendicular to the print axis.

FIGS. 6A to 6F show different examples of print heads according to theinvention.

FIGS. 6A and 6B show a longitudinal view and a transverse viewrespectively of a structure composed of a substrate S2 on which fourlayers K3, K4, K5 and K6 are stacked in sequence.

FIGS. 6C and 6D show two examples of print heads comprising structuresassembled using connections by solder beads. Those skilled in the artknow how to make connections by solder beads referred to as the“flip-chip” technique.

FIG. 6C shows an assembly of two structures. The first structure iscomposed of a substrate S3 on which two layers K7 and K8 are bonded insequence. A second structure is composed of a substrate S4 on which twosuccessive layers K9 and K10 are bonded. The connection by solder beadsis made between the free faces of layers K8 and K10. For example, thisstructure can give a resolution of the order of 1200 dpi.

The print head shown in FIG. 6D comprises the association of threestructures. A first structure is composed of a substrate S5 covered on afirst face by two layers K13 and K14, and on the other face by twolayers K15 and K16. The other two structures are each composed of asubstrate (S3, S6) on which two layers are bonded (K7 and K8 forsubstrate K3; K15 and K16 for substrate S6). The free faces of layersK16 and K14, and the free faces of layers K8 and K12, are assembled bysolder beads.

FIGS. 6C and 6F show two examples of print heads according to theinvention including diodes for control of the magnetic heads.

FIG. 6E shows a structure with two layers K17 and K18, each layercomprising a set of diodes D for controlling the magnetic headscontained in it. FIG. 6F shows a structure with two layers K19, K20. Adiodes block BD is connected by solder beads b to the free face of theupper layer K20.

FIGS. 6A-6F are given by way of non-restrictive examples. Moregenerally, as already mentioned above, the invention relates to anystructure comprising at least one assembly composed of a receptionsubstrate on which at least two layers of material, each provided withmagnetic heads, are deposited. The assemblies composed of a receptionsubstrate and layers of material can then be connected to each other bysolder beads or by any other means.

1. Structure comprising: a sequence of elements to send or receive asignal along an axis; two successive elements along a direction of theaxis being offset from each other along a direction perpendicular to theaxis; at least one set of at least two layers of semiconducting materialdeposited on a reception substrate, each layer of semiconductingmaterial comprising a sequence of elements in line with each other alonga direction parallel to the axis, two successive elements of thestructure along the direction parallel to the axis belonging to twodifferent layers, the layers of semiconducting material in at least oneset of layers being brought into bonding contact by molecular bonding.2. Structure according to claim 1, wherein a first assembly is connectedto a second assembly by solder beads.
 3. Structure according to claim 1,wherein at least one reception substrate is covered with at least twolayers of material on a first face and with at least two layers ofmaterial on a second face.
 4. Structure according to claim 1, furthercomprising at least one set of diodes in a form of a chip connected bysolder beads onto a face of a layer of material.
 5. Structure accordingto claim 1, further comprising at least one set of diodes in a form ofdiodes integrated into a layer of semiconducting material.
 6. Print headcomprising a sequence of magnetic heads or resistances to print along anaxis, including a structure according to claim 1, in which an element ofthe sequence of elements is a magnetic head or a resistance.
 7. Processfor manufacturing a structure including a sequence of elements to sendor receive a signal along an axis, the process comprising: assembling atleast two layers of semiconducting material, each comprising a sequenceof elements aligned along a direction parallel to the axis, the assemblybeing made such that two successive elements of a structure along adirection parallel to the axis belong to two different layers, theassembling comprising transferring a second layer of semiconductingmaterial onto a first layer of semiconducting material to bring a firstface of the first layer into bonding contact with a first face of thesecond layer, by molecular bonding.
 8. Process according to claim 7,wherein the second layer of material is transferred onto the first layerof material by a transfer support fixed to the second layer.
 9. Processaccording to claim 8, further comprising removing the transfer support,once the faces of the layers have been put into bonding contact. 10.Process according to claim 7, further comprising connecting using solderbeads between a first assembly composed of a first reception substrateonto which at least two layers of material are fixed by bringing theminto bonding contact, and at least one second assembly composed of asecond reception substrate, on which at least two layers ofsemiconducting material are fixed by bringing them into bonding contact.11. Process according to claim 7, further comprising connecting at leasta set of diodes onto at least one layer of material by solder beads,each diode in the set of diodes enabling control of an element of thelayer.
 12. Process according to claim 7, further comprising forming atleast a set of diodes in at least one layer of semiconducting material,each diode in the set of diodes enabling control of one element in thelayer of semiconducting material.
 13. Process for manufacturing a printhead including a sequence of magnetic heads or resistances to make aprintout along an axis, using a process according to claim 7, in whichone element of the sequence of elements is a magnetic head or aresistance.